The present invention relates to a resonant forward converter for switching a switching device in a state that applied voltage is substantially zero.
In recent years, to reduce switching power loss by junction capacitance of a switching device, a zero-voltage-switching technique for switching the switching device in a state that applied voltage is substantially zero has been applied to power converters. Prior art quasi-resonant converter with a resonance on the primary side and multi-mode resonant converter with resonances on the primary and secondary sides which carry out zero voltage switching can reduce switching power loss in the switching device. However, since voltage of the switching device becomes excessively high when load current increases, and resonant current in the circuit becomes heavy, the above converters need to use large capacity electronic parts including the switching device, resulting in a problem of increased power loss therein.
Referring to FIG. 8, this problem will be described in connection with a prior art resonant converter, wherein 1 designates a DC power supply, 2 designates a transformer including a primary winding 2A and a secondary winding 2B, each having the polarity as illustrated in FIG. 8, 3 designates a switching device comprising a MOSFET or a bipolar transistor, and 4 designates a resonant capacitor connected in parallel to the switching device 3. Resonant linear inductance is a sum of inductance Lr corresponding to a sum of inductance of wiring and leakage inductance of the transformer 2, and inductance of an individual linear inductor 5'. A rectifier diode 6 is connected in series to the secondary winding 2B of the transformer 2. A free wheeling diode 7 is interposed between one end of the secondary winding 2B of the transformer 2 and one end of the rectifier diode 6. An output filter comprising a smoothing inductor 8 and a smoothing capacitor 9 is interposed between the free wheeling diode 7 and output terminals 10, 10'. A resonant capacitor 11 is connected in parallel to the secondary winding 2B of the transformer 2. A control circuit 12 is arranged to provide control signals to the switching device 3 so as to maintain DC output voltage across the output terminals 10, 10', at a set value.
The prior art resonant converter, which is not described in detail here, is characterized that zero voltage switching is realizable in a wide current range without making voltage across the switching device 3 upon turning-off excessively high when load current increases. FIG. 9 shows typical operation waveforms of this resonant converter.
However, to obtain zero voltage switching as shown in FIG. 9, resonant current in the forward direction should be increased in response to a flow of resonant current in the backward direction, requiring heavy resonant current to thereby increase loss in the circuit, resulting in a decrease in efficiency. Particularly, the ratio of resonant current to output current increases upon low load, lowering power efficiency remarkably.
The greater a value of resonant linear inductance is, the smaller a value of the resonant capacitor 4 can be. Thus, resonant current can be reduced, whereas output power is also reduced. The ratio L/C should be set to a value corresponding to output power, and the resonant capacitor 4 should have large capacitance to meet a desired resonance operation and output characteristic, so that charging and discharging current of the resonant capacitor 4 becomes heavy as described above. Therefore, switching loss in the switching device 3 is fully reduced by zero voltage switching, whereas due to an increase in current passing through the switching device 3, resonant linear inductance, and the windings of the transformer 2, power loss is increased, lowering power efficiency.
Therefore, it is an object of the present invention to provide a resonant forward converter which contributes to improvement in power efficiency by realizing zero voltage switching in a wide range of load current without excessively high voltage across a switching device.
It is another object of the present invention to provide a resonant forward converter capable of adopting a small resonant capacitance with a largely reduced charging/discharge current passing therethrough, and reducing circulating current.